This invention relates to systems for ion implantation of semiconductor wafers and, more particularly, to methods and apparatus for alignment and calibration of ion beam systems using Faraday beam current sensors with small sensing apertures.
Ion implantation has become a standard technique for introducing conductivity-altering impurities into semiconductor wafers. A desired impurity material is ionized in an ion source, the ions are accelerated to form an ion beam of prescribed energy, and the ion beam is directed at the surface of the wafer. The energetic ions in the beam penetrate into the bulk of the semiconductor material and are embedded in the crystalline lattice of the semiconductor material to form a region of desired conductivity.
Ion implantation systems usually include an ion source for converting a gas or a solid material into a well-defined ion beam. The ion beam is mass analyzed to eliminate undesired ion species, is accelerated to a desired energy and is directed onto a target plane. The beam is distributed over the target area by beam scanning, by target movement or by a combination of beam scanning and target movement.
Ion implanters typically include components that deflect the ion beam using magnetic fields or electrostatic fields. For example, the ion implanter may include a mass analyzer which deflects different ion species in the ion beam by different amounts. In addition, the ion implanter may include an electrostatic or magnetic scanner for deflecting the ion beam over the surface of the wafer being implanted. Furthermore, ion implanters commonly include angle correction magnets that convert a scanned ion beam with diverging ion trajectories into an ion beam with parallel ion trajectories. A typical ion implanter includes several components that deflect the ion beam as described above. The beam deflections must be carefully controlled in order to ensure uniform and efficient implantation of the target wafer. Deviations of the ion beam from the desired path to the target may result in sputtering of beamline components, target contamination and reduced beam current delivered to the target.
It will be understood that inaccuracies and variations in the magnetic and/or electric fields used to deflect the ion beam result in deviations of the ion beam from the desired beam path. The deflection of an ion beam passing through a magnetic field is a function of the magnetic field strength, the distance over which the magnetic field is applied, the ionic mass, the ionic charge and the beam energy. Thus, differences in magnet geometry and magnetic field may cause the ion beam to deviate from the desired ion beam path. Furthermore, it is customary to utilize the ion implanter to implant ions of different species, different energies and different charge states. When the parameters of the ion beam, such as ion species, ion energy and the like, are changed, it is necessary to adjust the magnetic and/or electric fields to ensure that the ion beam follows the desired ion beam path. Where the ion implanter has several beam deflection components, the alignment of the ion beam can be difficult and time consuming. In addition, realignment is necessary each time the beam parameters are changed. In view of the foregoing, there is a need for improved methods and apparatus for alignment and calibration of ion implanters.
A beam current sensor in the form of a Faraday cup, or Faraday current detector, is typically used to measure ion current in an ion implanter. A Faraday cup includes an electrode mounted in a conductive enclosure and electrically isolated from ground. An ion current entering the enclosure produces an electrical current in a lead connected to the electrode. The electrical current is representative of the ion current.
U.S. Pat. No. 4,922,106 issued May 1, 1990 to Berrian et al discloses the use of a translating Faraday current detector to determine dose uniformity over the area of the wafer. U.S. Pat. No. 4,751,393 issued Jun. 14, 1988 to Corey, Jr. et al discloses the use of multiple Faraday cups disposed around the periphery of the wafer to determine dose uniformity.
According to a first aspect of the invention, a method for sensing an ion beam is provided. The method comprises the steps of generating an ion beam and directing the ion beam along a beamline, and sensing the ion beam with a beam current sensor positioned on or adjacent to the beamline. The beam current sensor has a sensing aperture that is smaller than the cross-sectional dimension of the ion beam at the current sensor, wherein the sensed ion beam current is indicative of ion beam position relative to a desired ion beam path.
The method may further include the step of adjusting the ion beam position if the sensed ion beam position differs from the desired ion beam path. Where the beam current sensor is positioned on the desired ion beam path, the ion beam position may be adjusted for maximum sensed ion beam current. The ion beam position may be adjusted by adjusting a magnetic field or an electric field applied to the ion beam, or by adjusting the position of an electrode or other component that affects ion beam position.
According to another aspect of the invention, the ion beam may be sensed with a plurality of beam current sensors located at different positions on or adjacent to the beamline. Each of the beam current sensors has a sensing aperture that is smaller than the cross-sectional dimension of the ion beam at the respective current sensor, wherein the ion beam current sensed by each of the beam current sensors is indicative of ion beam position relative to a desired ion beam path.
According to another aspect of the invention, a method is provided for calibrating a system element in an ion beam system, wherein the system element changes the position of an ion beam with respect to an ion beam path depending on a parameter Y of the system element and a characteristic X of the ion beam. The method comprises the steps of positioning a beam current sensor on or adjacent to the ion beam path, and, using the beam current sensor, determining a relation Y=f(X) between the characteristic X of the ion beam and the parameter Y of the system element that is required to direct the ion beam along the ion beam path. The beam current sensor has a sensing aperture that is smaller than a cross-sectional dimension of the ion beam at the beam current sensor.
For a magnetic system element, the characteristic X comprises the magnetic rigidity of the ion beam, and the parameter Y comprises the magnetic field produced by the system element. For an electrostatic system element, the characteristic X comprises the energy and the charge state of the ion beam, and the parameter Y comprises the electric field produced by the system element.
The relation Y=f(X) may be determined by measuring two or more sets of values of the characteristic X and the parameter Y required to direct the ion beam along the ion beam path. For a beam of characteristic X, the parameter Y is adjusted for maximum sensed beam current. The relation Y=f(X) can then be utilized to set the parameter Y so as to direct an ion beam of characteristic X along the ion beam path.
In a more specific case, a method is provided for determining a relation between magnetic rigidity R of an ion beam and magnetic field B required to direct the ion beam along a desired path in an ion beam apparatus. The method comprises the steps of positioning a beam current sensor on or adjacent to the desired path, using the beam current sensor to determine a first magnetic field B1 required to direct a first ion beam having a first magnetic rigidity R1 along the desired path, and using the beam current sensor to determine a second magnetic field B2 required to direct a second ion beam having a second magnetic rigidity R2 along the desired path. From the values of B1, B2, R1 and R2, values of a0 and a1 in the equation B=a1 R+a0 are calculated, thereby providing a relation between magnetic rigidity R of an ion beam and magnetic field B for the ion beam apparatus.
According to yet another aspect of the invention, an ion implanter is provided. The ion implanter comprises an ion source for generating an ion beam and for directing the ion beam along a beamline toward a target position, an ion beam deflection element disposed along the beamline for deflecting the ion beam relative to the beamline, and a beam current sensor positioned on or adjacent to the beamline for sensing ion beam current. The beam current sensor has a sensing aperture that is smaller than the cross-sectional dimension of the ion beam at the current sensor, wherein the sensed ion beam current is indicative of ion beam position relative to a desired ion beam path. The ion implanter further comprises means for adjusting the ion beam deflection element, in response to the sensed ion beam position, so that the ion beam is directed along the desired ion beam path.